There are five known conventional phases, or states, of matter: solids, liquids, gases, plasma and condensates (e.g., Bose-Einstein-condensates). These differ in their densities, energies, and the nature of the interactions between their constituent particles. At the extremely low end of the temperature-energy scale, atomic motion comes very close to stopping altogether as most of the atoms all fall into their minimum energy ground state. Since there is almost no kinetic energy or relative motions between the atoms they can self-organize—or condense—on the basis of the weakest, quantum mechanical interactions that occur when their separation corresponds to their Debroglie wavelength that is almost 100 times larger than the distance between the molecules in air. When this occurs the system exhibits such quantum mechanical effects as interference and superfluidity, making a Bose-Einstein condensate (BEC) a platform to study quantum mechanics and its particle-wave paradox on a macroscopic level. Light can be slowed down to a few cm/sec. The superfluidity of a BEC allows it to flow without friction, so that in the absence of dissipation it will persist essentially forever.
As such, enabled by the development of laser cooling methods into the nanoK regime, a dilute atom-gas Bose-Einstein condensate of Rb was first realized in 1995, 70 years after the prediction of this unusual state of matter by its namesakes, an achievement that awarded the 2001 Nobel prize for physics to Cornell and Weiman(1) and Ketterle(2).
Prior to this, however, predictions had also been made for condensates composed of certain so called “quasiparticles” that are typically excited states of more conventional atoms and electrons or combinations of these conventional particles with energy in some form. These include excitons(3-5) and phonons involving dipoles(6, 7), based on inhibited relaxation that gave the requisite athermal population distribution. Although the difficulties in making BCS superconducting and dilute atomic gas Bose-Einstein condensates could imply that condensation is a fragile process, it could well be the opposite. Following these predictions, such non-equilibrium condensates have been produced with a number of constituents, e.g., photons, excitons(8, 9), magnons(10, 11), exciton-polaritons(12, 13), and more. The hosting of these non-equilibrium condensates of quasiparticles inside solid hosts and, because of the low masses of their constituent (quasi) particles and the intrinsically higher temperatures associated with excitations, their persistence to ambient temperatures in some cases(14) make them much more amenable to both study and potential applications(15-17).
A conspicuous absence from this quasiparticle menagerie is polarons. This is especially important because polarons are the origin of the properties of doped transition metal oxides and chalcogenides, most notably their exotic superconductivity. In addition, unlike the other quasiparticles, polarons possess charge, localized spin, electrons in specific orbits, and mass, and are therefore much more receptive to our abilities to manipulate electricity, magnetism, and matter that form the basis for our microelectronics, optics, magnetics, and related technologies. The discovery that charge inhomogeneities in UO2 resulting from either static doping with excess Oxygen or transient photoexcitation via the metal-to-metal charge transfer transition aggregate and self-organize to form a coherent polaron quantum phase (CPQP) that exhibits extraordinary coherence and either is a condensate or possesses many of the properties(18-20) of the non-equilibrium type predicted by Fröhlich in his 1968 paper. (6, 21, 22) suggests that polarons should be further studied.
Pertinent to the polaron, regarding the Fröhlich BEC problem, a large number of unusual or unique results from structural and spectroscopic experiments on the O- and photodoped 5f Mott insulator, UO2(+x) that indicate its presence in this system have been reported (18-20). As used herein UO2+x is UO2 containing extra O (oxygen atoms) so that it is a combination of U(IV) and U(V) that retains its original fluorite structure through x=0.33−0.5. As used herein, UO2(+x) is photoexcited UO2, the excitation here is from the highest occupied U 5f state to the lowest unoccupied U 5f or 6d states on a different U ion, this metal-to-metal charge transfer transition creates U(III) and U(V). UO2(+x) can also mean UO2+x and vice versa, referring to mixed valence UO2 with charge defects or inhomogeneities created by both chemical methods and photoexcitation. These can also mean UO2 without or prior to the creation of the charge defects by these means. The combination of XAFS, x-ray pair distribution function (pdf), and neutron pdf showed tunneling polarons (18) whose displacements are far too large for conventional tunneling, exceeding those in cuprates (23-29) by at least an order of magnitude. These tunneling polarons aggregate and self-organize into the relevant coherent, polaronic, quantum phases, possessing exceptional coherence, stability, and other collective effects persisting even up to ambient temperature. Their non-equilibrium condensate-like properties would then be the culmination of the phonon-coupled, synchronous charge transfer displayed by related systems (30, 31). These comprehensive structural results indicate that the coherence of the CPQPs is possibly enhanced by a Fano-Feshbach (19, 20, 32-35) or other types of resonances that connects the U(IV,V) ground state closed channel and U(IV,VI) excited state open channel species that are preferred at the opposite ends of the vibrational excursion of a special [111] phonon (36). Although time domain optical pump-optical and THz probe experiments on UO2.0 and electron paramagnetic spectroscopy measurements on O-doped UO2+x have identified a number of non-UO2 states with extraordinary coherence and collective properties, an outstanding question has been the electronic structure.
Conradson et al., 2013, “Possible Bose condensate behavior in a quantum phase originating in a collective excitation in the chemically and optically doped Mott-Hubbard system, UO2(+x).” Physical Review B, 88 115135 (Conradson 2013) discloses through U L3 EXAFS and x-ray pair distribution function experiments performed at the Stanford Synchrotron Radiation Lightsource combined with neutron pair distribution function experiments performed at Lujan Neutron Scattering Center that: x-ray and neutron structural probes demonstrate tunneling polaron in UO2+x, but more radical than the one found in cuprates. Conradson 2013 speculates that it was superfluid tunneling, although superfluid tunneling of atoms inside a crystal has never been postulated. Further, Conradson 2013 discloses O XAS experiments on UO2+x performed at SSRL and Non-resonant Inelastic X-ray Scattering experiments performed at the Advanced Photon Source in which the electronic levels of UO2+x did not fit the pattern set by related compounds. Conradson 2013 postulates that this originated in the tunneling polaron in UO2+x. Conradson 2013 further discloses Raman experiments on UO2+x performed at the Environmental Molecular Science Laboratory in which: UO2 in powder form did not scatter, UO2+x gave weak scattering with a very broad spectrum but not the feature associated with the U(VI)-oxo group found in the x-ray measurements. Conradson 2013 postulated that this originated in the tunneling polaron in UO2+x. Conradson 2013 further discloses ultrafast time domain optical laser pump-optical laser reflectivity probe experiments on UO2 crystal performed at Los Alamos National Laboratory in which extraordinarily long lifetimes were found, indicating something is stabilizing the excited state or inhibiting its relaxation. The observed complicated temperature dependence of lifetimes and time domain spectra indicate that the photoinduced polarons form a quantum phase that is separate from the host UO2 and has multiple states, one of which undergoes a gap opening phase transition around 50-60 K. Conradson 2013 speculated that these states would be consistent with a Bose-Einstein condensate. Conradson 2013 further discloses ultrafast time domain optical laser pump-THz probe spectroscopy experiments on UO2 crystal in which it was observed that some states were changing their relative populations at lower temperatures although the UO2 phase diagram shows no additional structures or states in this temperature range. Conradson 2013 concluded that UO2+x and UO2(+x) contain polarons with unique or unusual properties, one of which is that the ones created by lower energy photoexcitation into the U 5f states aggregate to form a relatively stable quantum phase separate from the host that can undergo a phase transition. Conradson 2013 speculated that these properties derive from the quantum phase being a coherent porlaon quantum phase that may be a non-equilibrium Bose-Einstein or similar condensate.
While much has been learned regarding O- and photodoped 5f Mott insulators, what is needed in the art are ways to apply UO2(+x) to electrical applications such as optical switches, electric and magnetic switches, transmission lines, ultrafast detectors, ultrahigh frequency circuits, magnets, motors, magnetic sensors, quantum computing, quantum communications, and energy storage.